There are currently recognized to be 16 human papillomaviruses and 6 bovine papillomaviruses. Each of these viruses is associated with distinct clinical entities which in humans include common warts, condylomata accuminata, laryngeal papillomatosis, and the macular pityriasis-like lesions of epidermodysplasia verruciformis. In cattle, these lesions are associated with cutaneous fibro-papillomas and esophageal papillomatosis. To date, no tissue culture system has been developed to propagate the papillomaviruses. There is a subset of papillomaviruses which are associated with carcinomas in their natural hosts. Among the human papillomaviruses, these include the HPV-5 and HPV-6; in cattle it includes the BATV (BPV-4) in cattle which feed on bracken fern. In the laboratory we have been studying the molecular biology of the BPV-1, because it can be used to transform susceptible rodent cells in tissue culture. Transformation of rodent fibroblasts by papillomaviruses remains one of the only biologic systems available to study the biology of papillomaviruses.